1. Field of the Invention
The invention relates to a coated object comprising a substrate and at least one functional layer with an optical function and/or a protective function, and to processes for producing a coated object of this type and also to the uses of this object.
2. Description of Related Art
Coated objects of this type have been known for many years and are indeed in widespread use. For many applications, it is necessary to apply very thin layers, in ranges of up to a few micrometers. These coatings may have a very wide range of different functions. So there are numerous very thin protective layers for protecting the object from, for example, mechanical loads, in the form of “scratch-resistant layers”, etc. These layers are often also supposed to have an optically attractive design and/or to be completely transparent. A further wide range of applications for coatings is formed by optical coatings having numerous functions and applications.
However, with many objects of the type described in the introduction, problems arise if they are exposed to operating temperatures of higher than approx. 350° C.
Reflectors, for example, have an operating temperature of approximately 400 to 500° C., while cooking hobs have operating temperatures of up to 800° C.
At these high temperatures, layers applied in amorphous form change. As the temperatures rise, the layers undergo a phase transformation, which is highly detrimental to the functioning of the coated objects. For example, in the case of TiO2, the phase transformation is from the amorphous phase to the crystalline anatase phase and then from the anatase phase into the rutile phase. This phase transformation involves volumetric shrinkage, which has extremely adverse effects on the overall layer assembly. On account of the volumetric shrinkage, microcracks are formed within the layer. By way of example, in the case of reflectors these microcracks then scatter the incident radiation, with the result that the maximum reflected light flux which can be achieved is reduced. The reflectivity of the coating is also reduced. The surface structure of the individual layers also changes as a result, which in the case of alternating layer systems may have further adverse effects, such as for example (partial) delaminations.
It is known, in order to solve this problem, to apply the layers directly in crystalline form. However, these layers are often rough, somewhat opaque and have worse reflection characteristics.
One further possible way of avoiding the problem is to dope a functional layer with an SiO2 glass-forming agent. A drawback of this process is that the functional layer has a lower refractive index and that the procedure is more complex to carry out. More or thicker layers and, as a corollary effect, more time to build up the layer assembly are required to achieve the same optical effect (reflection).
There are studies on phase transformations and crystallization properties of thin (alternating) layers. For example, it is known from the literature [H. Sankur and W. Gunning, J. Appl. Phys. 66 (1989)] that the crystallization properties are dependent on the one hand on the temperature but also on the other hand on the layer thickness of the individual layers. The thicker the layers are and the more the layers are heated, the more likely crystallization is to occur.
H. Sankur and W. Gunning discovered that layers of TiO2 crystallize from the amorphous phase to the anatase phase at approximately 350° C. and then crystallize into the rutile phase at approximately 600° C. and above. Their studies have shown that with very pure layers, below a certain layer thickness, namely less than approx. 50 nm, the crystallization is made so much more difficult, even at relatively high temperatures, that in actual fact it simply does not occur.
A further study on this topic by D. G. Howitt and A. B. Harker [J. Mater, Res. 2 (2), March/April 1987] shows that relatively thin layers (<50 nm) do not crystallize, i.e. remain in the amorphous state, even if they are heated for a relatively long time at elevated temperatures. For example, heating a thin layer (<50 nm) at 450° C. for 100 hours did not reveal any crystallization. Under these conditions, thicker layers would crystallize within a few minutes.
With a view to the optical design, however, it is generally necessary to apply layers which have a thickness of greater than 50 nm. In this respect, the optical design stipulates a certain layer structure, i.e. an accurately defined sequence of the physical layer thicknesses d in the alternating layer assembly, the values of which thicknesses may typically be up to 200 nm for applications in the visible spectral region. The corresponding optical layer thicknesses n-d are of the order of magnitude of λ/4 (n: refractive index, λ: light wavelength).
Furthermore, it has been found that with the known processes it is impossible to produce sufficiently thermally stable layers which are hard, dense and scratchproof, in particular if they are also supposed to have an attractive optical appearance in the long term and/or have to be transparent.
By way of example, it is known from DE 42 01 914 A1 (=U.S. Pat. No. 5,594,231) to provide scanning windows made from glass or glass-ceramic for scanning systems installed in checkouts of supermarkets and other consumer markets to record barcodes which have been applied to product packaging, with a light-transmitting hard-material layer on the top side, and in turn to apply a light-transmitting coating with sliding properties to the hard-material layer, in order to make these scanning windows more resistant to wear. Materials which may be mentioned for the hard-material layer include, inter alia, metal oxides, such as Al2O3, ZrO2, SnO2, Y2O3. Aluminum oxide deposited in amorphous form is mentioned as being particularly suitable. In particular the amorphous deposition of the metal oxide in this context promotes the desired improved hardness and sliding properties of the protective layer. The hard-material layers described here are suitable for applications in the region of room temperature, but their properties change at high temperatures, as are customary, for example, in cooking areas, making them unsuitable for use at high temperatures. A protective layer for cooking areas requires materials which are able to withstand temperatures of up to 800° C. and which are also able to withstand the high thermomechanical stresses which occur between the glass-ceramic and protective layer.
DE 201 06 167 U1 has disclosed a cooking area having a glass-ceramic plate as the cooking plate, which is provided with a transparent scratch-resistant layer which may be formed, inter alia, by a hard-material layer. Materials which are mentioned for this transparent layer include, inter alia, metal oxides, such as aluminum oxide, zirconium oxide, yttrium oxide, tin oxide, indium oxide and combinations thereof. According to this document, the materials can be deposited, for example, by the sol gel technique, the CVD processes, in particular the PICVD process, and by sputtering.
With the known processes for producing hard-material layers, as described, for example, in the abovementioned documents DE 42 01 914 A1 and DE 201 06 167 U1, the layers are typically deposited in amorphous form or in a partially crystalline structure. In the event of prolonged use in the hot areas or in the event of their maximum thermal load being reached, layers of this type may experience disadvantageous changes. For example, in these areas the layers may be discolored by thermally induced compacting or may become partially opaque through crystallization, with the result that the hot areas become visually noticeable. Furthermore, roughening in the range from 1-1000 nm may occur. The roughening itself may already make its optical presence felt, with the recesses which are formed additionally making them more difficult to clean. The problem of crystallization in the hot areas intensifies mechanical failure mechanisms of the scratch-resistant layer. During crystallization, the structure of the layer changes, so that cracks are formed in the layer. The loss of lateral cohesion means that the layer no longer offers sufficient protection against scratching.
It is known from the application area of turbine technology that layers which are grown in pillar form have a particularly high resistance to rapid alternating thermal loads. For example, U.S. Pat. No. 4,321,311 describes the use of a ceramic layer grown in columnar form as thermal protection for metallic components of the turbine structure. However, the layers described in this document have a considerable roughness and/or porosity, on account of their coarse crystalline structures.
Rough and porous surfaces quickly become dirty and are difficult to clean. Moreover, in optical terms they are not fully transparent, but rather have a strong scattering action and are unsuitable for applications with optically attractive surfaces.